In recent times, satellite-based time signals are being increasingly emitted in addition to terrestrially emitted time signals, e.g. DCF-77. The most well known methods are the GPS system and the GLONASS system.
A serious disadvantage is the necessity of highly accurate satellite positioning and exact knowledge of the transmission path, especially of the ionosphere and troposphere, which is indispensable to a user requiring maximum accuracy. In addition, the satellite signals are deliberately corrupted for civilian users (“selective availability”) in order to prevent non-military utilization requiring maximum accuracy. Methods have been developed which allow for partial compensation to these uncertainties (e.g. differential GPS). The difficulties relating to using the GPS signal for high-precision time applications have so far not been satisfactorily solved.
The said methods are widely used because of the inexpensive availability of suitable receiving devices. An operational disadvantage is seen in just this military nature of the systems which impede industrial utilization. Satellite-based time signals require an extensive infrastructure for monitoring and verification. A further disadvantage is that high-precision data are available only with time delays of hours or longer from the said systems.
The two-way method (TWSTFT, Two-Way Satellite Time and Frequency Transfer) for time transmission is particularly suitable for metrological purposes. It is a method used by national calibration authorities (e.g. PTB Brunswick) for comparing existing time scales based on atomic clocks.
The advantage of this method lies in the basic independence of satellite position and of errors due to the transmission path. It can be derived directly from the symmetry of the method. Since both connection partners require both a transmitting and a receiving device, the application of the method is restricted to a few national authorities (DE, GB, FR, OE, US, IA, IT, ES, NL) because of the relatively high costs. Different transmission methods can be used: FDMA (Frequency Division Multiple Access), CDMA (Code Division Multiple Access) or TDMA (Time Division Multiple Access), and the multiplex method in which                the remote ground station can be connected to a system of redundant central clocks        an arbitrary number of remote ground stations can be connected to the central clock                    an arbitrary number of remote ground stations can be connected to a redundant system of central clocks.                        
The increasing availability of small inexpensive satellite ground stations with transmitting device now pushes the system-related disadvantages more and more into the background. It seems natural to make the two-way method, which has been successful for years, accessible to widespread use as an alternative to one-way methods (GPS, GLONASS).
A barrier to this has previously been that the 2-way method, also called TWSTFT (Two-Way Satellite Time and Frequency Transfer) was restricted to the comparison of existing clocks located externally to the devices described here and that the measurement results are only published with a time delay of up to several days after corresponding calculations by the BIPM (Bureau International des Poids et Mesures, Paris).